Pacific ENSO Update - August 1995 - Vol.1 No.3

CURRENT CONDITIONS

The last issue of Pacific ENSO Update reported on the changing trends in sea surface temperatures (SSTs) and the Southern Oscillation Index (SOI -a measure of atmospheric surface pressure patterns) in the tropical Pacific. Taken together, these trends indicated an end to the multi-year (1991-95) ENSO warm event. Although this particular event has apparently ended, Pacific ENSO Update will continue to be issued on a quarterly basis, to summarize ongoing climate conditions in the Pacific region and inform on the impacts of "normal," as well as "warm" and "cold" phases, of the ENSO climate cycle...

In the last few months, SSTs and the SOI have continued in their trends toward near-normal values. Specifically, SSTs have returned to within 0.5 degrees C of their normal ranges throughout the equatorial Pacific, and the SOI has continued to rise toward its long-term average value of zero. While there are some exceptions, most ENSO forecasting models are predicting at present that both the SOI and SSTs will remain near their normal values, at least for the next 9 to 12 months. However, some unusual atmospheric conditions, which occurred over the tropical Pacific in June and July, appear to have contributed to an extended period of below-normal rainfall, particularly in the western tropical Pacific, affecting the near-term outlook in several regions. The secton on page 2 entitled "What's a TUTT?" gives some more details on the impact of these conditions...

WHAT'S A "TUTT"?

Consistent with post-ENSO trends, below-normal rainfall was experienced during the first half of 1995 throughout much of Micronesia, including the Mariana Islands, the northwestern Caroline Islands, and Palau. In June and July however, the normally heavy summer rainfall expected for Micronesia was reduced, likely due to unusual atmospheric conditions which occured during those months: Two high-altitude westerly wind patterns - the "mid-latitude jet stream" in the Southern Hemisphere, and the "TUTT" (or "tropical upper tropospheric trough" - a general circulation feature found at high altitudes over tropical and subtropical ocean areas in summer) in the Northern Hemisphere - were both located closer to the equator than normal. Convergence of these two high-altitude westerly wind currents over Micronesia had several impacts on normal summertime weather patterns there, including:

increased subsidence, or downward movement of air toward the surface, working against the normal formation of deep clouds and their heavy, long lasting showers;

increased wind shear, or difference in wind directions between low and high altitudes, that tears the tops off of deep clouds and prevents moisture from rising high into the atmosphere - a necessary condition for heavy rain;

stronger-than-normal easterlies, or trade winds, in the western tropical North Pacific, preventing the low-altitude westerly winds of the normal monsoon season from pushing into the region and supplying moisture for rainfall and tropical cyclone development.

All these conditions contributed to reduced rainfall and limited tropical cyclone activity throughout Micronesia. In fact, these impacts are similar to the kinds that a La Niña event (the "cold" phase of the ENSO cycle) would have on the region. At present however, trends in SSTs and SOI do not indicate a developing La Niña. These unusual atmospheric conditions are instead expected to be only a temporary feature in a trend toward more normal rainfall and storm activity in the coming months. The situation does however add a new twist to our post-ENSO outlooks.

LOCAL VARIABILITY SUMMARIES:

As noted in each issue of Pacific ENSO Update, the following summaries of expected local climate variability for the various island areas indicated are not based on official forecasts (unless otherwise noted), but rather draw on: 1) statistical information about past associations between local climate variabilities and the ENSO cycle, 2) the advice of research staff of the Pacific ENSO Applications Center, 3) various experimental forecasts, and 4) other sources. Credits for the individual contributions in this section are also included.

HAWAII: Summertime rainfall in the Hawaiian Islands is strongly influenced by the character of trade winds experienced during the season. Trade wind rainfall brings showers to windward and mauka island areas, while leeward areas remain dry until the appearance of autumn frontal (Kona) storm systems, beginning in the fall. Early summer trade winds were weaker than normal this year, primarily due to a northward shift in the position of the subtropical high (an atmospheric pressure system which generally delivers consistent trade winds to the islands throughout the summer). More recently, the trade winds have returned and mountain stations have begun to receive normal amounts of rainfall. Upcountry Maui has had sufficient rains to alleviate the water restrictions which had been in place there. However, the "rainy season" for the Kona district of the Big Island is only just now beginning. Historically, September is the wettest month for Kona. Total rainfalls for August and September will be critical to alleviating the extended drought conditions which have been experienced there. The outlook for rainfall in the coming months is for a gradual return to near-normal conditions throughout the islands. This is consistent with the longer-term influence of the post-ENSO return to normal sea surface temperatures (SSTs) and atmospheric circulation patterns in the region. The latest issue of the Long-Lead Outlook for the Hawaiian Islands from NOAA-CPC (see Appendix I) generally reflects this trend.

Hurricane activity in the central tropical Pacific has been quiet thus far. In other recent post-ENSO hurricane seasons (e.g.1983,1988), July or early August had seen at least one system enter the region of concern for Hawaii (west of the 140 degrees W longitude line, or about 900 nautical miles from the Big Island). Except for the short-lived venture of tropical storm "Barbara" into the region (July 16-17), no systems of any consequence have threatened Hawaii. The absence of influence by the subtropical high has likely been a key factor in reducing nearby storm activity for the early season, as has the post-ENSO cooling trend in sea surface temperatures south and east of the islands. The total number of storms in the eastern tropical Pacific (east of 140 degrees W) have been near-normal for the season thus far. - sources: NOAA-NWSPR-WSFO, NOAA-CPC, and U.H. Dep't of Meteorology

AMERICAN SAMOA: American Samoa lies in a region between the extreme impact areas of the ENSO cycle on rainfall in the Pacific. Warm events generally cause wet conditions to occur north and east of the islands, and dry conditions to the south and west, with the result that local impacts on rainfall in American Samoa are small and variable. However, the impact of ENSO on tropical cyclone distribution patterns in the South Pacific is somewhat more important - two of the last three hurricanes to impact Samoa occurred during years with ENSO warm events (Tusi [1/87] and Val [12/91], with Ofa [2/90] occurring during a "normal" year). The recent abatement of ENSO warm conditions should reduce the chance of tropical cyclone and storm impacts in the region. Activity for the coming 1995-1996 season should be centered further west, toward more normal longitudes between Vanuatu and Australia. Sea surface temperatures along the equator north of Samoa have stabilized in recent months near their normal values, and are expected to remain so in the coming months.- source: UOG-WERI

GUAM/CNMI: Rainfall from November 1994 through July 1995 has been about two-thirds of normal on Guam and in the CNMI. On Guam, recent rainfall has been sufficient to sustain lush vegetation and has stabilized the falling water levels in the Fena reservoir. The Navy has lifted the water conservation measures for the remainder of the year. In Saipan, the rain has also been sufficient to keep the island green.

The slightly below normal rainfall predicted in the last issue of Pacific ENSO Update, for June and early-to-mid July, was accurate. However, the meteorological conditions that usually produce heavier summertime rains in Guam and the CMNI appear to be about one month late in setting up. This has extended drier-than-normal conditions into mid August. The normal monsoon pattern may not become well established until it moves south of the region in September. Because of this delay, total wet season rainfall for Guam and the CNMI is now expected to be slightly (about 10-20%) below normal. However, a single tropical storm or typhoon event could push wet-season rainfall above normal.

Per their request, the following expectations of monthly rainfall totals are provided for the use of water resource managers on Guam:

The outlook for the remainder of the current typhoon season is for near-normal activity. Per the normal pattern, tropical cyclone activity began in the Philippine Sea in June. However, the number of storms has been very low (as of August 12th, the Atlantic Ocean has seen more storms than has the western North Pacific--a rare occurrence!). Activity is expected to pick up in late August, and slowly move eastward as the year progresses. Typhoon threats to Guam and CNMI will be greatest from late September through mid-December.- source: UOG-WERI

MICRONESIA: The summary for FSM is split into two regions south and north of 9 degrees N latitude (see map).

Region 1 (south of 9 degrees N): May-July rainfall at Pohnpei was only 73% of normal, and rainfall at Chuuk for the same period was 81% of normal. The failure of the normal monsoon pattern to set up in this region is responsible for these shortfalls (see the "What's a TUTT?" section on p.2). Below-normal rainfall (75-80% of normal) may persist in the region through August and possibly into September, with eastern areas like Chuuk possibly seeing some more rain than Pohnpei or Kosrae. However, rainfall is expected to return to normal for all areas beginning in September, when the monsoon conditions become established. Normal patterns of typhoon activity can be expected for all FSM states from October through mid-December. However, with the disappearance of ENSO warm conditions, typhoon threats in eastern areas like Kosrae and Pohnpei State are expected to be greatly reduced. The extended outlook for January - June 1996 rainfall is for normal to slightly-dry conditions.

Region 2 (north of 9 degrees N): Because of the delay in appearance of the normal monsoon pattern, rainfall at Yap for May through July was only 65% of normal, with June and July seeing less than half of normal rainfall. Drier-than-normal conditions may persist in the northern part of the FSM through August or early September, with the islands and atolls receiving about 75% of normal rainfall. After late August, near-normal conditions should return, but total rainfall for 1995 may only be about 80% of normal due to the early shortfalls. Typhoon activity in the northern FSM can be expected from mid-September through mid-December. The extended outlook for January to August 1996 rainfall is for normal to slightly-above-normal rainfall.- source: UOG-WERI

PALAU: While May was wet for Palau (139% of normal), June and especially July were quite dry, giving a total rainfall for the three-month period at only 83% of normal. The delay in the appearance of the normal monsoon pattern led to these dry conditions. Rainfall is now expected to slowly increase, becoming near-normal by early to mid September, but because of shortfalls early in the year, total rainfall for 1995 may only be about 80-90% of normal. Tropical storm and typhoon activity in the region is expected to increase by September, with greatest risks for Palau occurring in October, November, or the first half of December. The extended outlook for January to August 1996 is for normal rainfall and storm threat conditions.- source: UOG-WERI

MARSHALL ISLANDS: With the disappearance of ENSO warm conditions, rainfall patterns in the RMI have returned to normal. Rainfall at Majuro for the three-month period of May, June, and July was 86% of the long-term average (35.77 inches). The 86% value is just about normal, since non-ENSO rainfall in the RMI comes primarily from trade wind showers, and should be somewhat less than the rainfall during ENSO years (which are included in the long-term averages).

Typhoon threats for the current season should be at their normally low levels in the RMI, especially for the eastern islands, since sea surface temperatures have cooled and wind patterns have changed with the end of ENSO warm conditions. The western islands may see some westerly winds and tropical storm activity in October and November, but by mid-December and January, trade winds should strengthen and prevent late-season typhoon development. Enewetok and Wake Island may experience some typhoon threats in September and October. The extended outlook for January through August 1996 is for near-normal (non-ENSO) rainfall and below-average tropical storm and typhoon activity.- source: UOG-WERI

ACKNOWLEDGEMENTS and FURTHER INFORMATION:

The information contained in the LOCAL VARIABILITY SUMMARIES section and elsewhere in this issue of the Pacific ENSO Update has been drawn from many sources. Further information may be obtained by contacting the individuals and institutions listed below:

NOAA National Weather Service CLIMATE ANALYSIS CENTER (CAC) Diagnostics Branch, and CLIMATE PREDICTION CENTER (CPC):
World Weather Building, Washington D.C. 20233.
Contact CAC at 301-763-8227 for more information on the ENSO Advisory.
Contact CPC at 301-763-8167 for more information on the Long-Lead Outlook for the Hawaiian Islands.

University of Guam (UOG) WATER AND ENERGY RESEARCH INSTITUTE (WERI):
Lower campus, University of Guam
UOG Station, Mangilao, Guam 96923
Contact C. Guard or M. Lander at (671)734-3132 for more info on tropical cyclones and climate in the Pacific Islands.

University of Hawaii (UH) School of Ocean and Earth Science and Technology (SOEST) DEPARTMENT OF METEOROLOGY:
HIG #331, 2525 Correa Road, Honolulu, Hawaii 96822
Contact Dr. T. Schroeder, Chairman, at 808-956-7476 for more information on hurricanes and climate in Hawaii.

Special Section - Electronic ENSO Information:

Below are reproductions of computer screens with information about average sea surface temperatures (SSTs) and winds in the equatorial Pacific. This information, which is collected by a network of scientific buoys (the "TOGA-TAO" network) and transmitted via satellite to NOAA's Pacific Marine Environmental Laboratory (PMEL) in Seattle, Washington, is updated daily and available by computer over the global network commonly called the "internet". This is only one example of the many kinds of real-time ENSO information which can be routinely accessed in this way. This particular view requires a special kind of computer software called a "browser" (e.g. Netscape or Mosaic), but can also be accessed by computer in other ways, for example through the PEACESAT communications network that is available in many Pacific island areas.

FIGURE 1: The top image shows the actual average, or "mean", sea surface temperatures (in degrees C) and winds observed over the equatorial Pacific Ocean, from 90 degrees W (near South America) to 140 degrees E (near New Guinea, which appears at left), for a five-day period ending on the 23rd of August, 1995. In the image below, the differences, or "anomolies", between the actual values and the long-term averages for the same period are shown. Notice that all temperatures and winds are close to normal, with slightly (about 0.5 degrees C) cooler water along the equator in the eastern and central Pacific, and slightly warmer in the west, north of New Guinea. The similar figures on page 6 provide a comparison between conditions during July 1995 (which were also close to normal) with those that were observed in December 1994 (when the ENSO warm event was near its peak strength).

FIGURE 2: These images show the "mean" or actual sea surface temperatures and winds observed over the equatorial Pacific Ocean during the month of July 1995 (top), and the "anomolies" or differences in those observations from long-term averages for July (bottom). Most temperatures and winds were very close to normal, with slightly (about 0.5 degrees C) warmer waters in the west near New Guinea. In general, these close-to-normal conditions are a good indication that the recent ENSO warm event, and its unusual pattern of sea surface temperatures and winds, have truely abated. For comparison, the situation observed in December 1994 is shown below in Figure 3.

FIGURE 3: These images show the conditions that were observed in December 1994, when the most recent ENSO warm event was near its peak strength. A comparison of the "anomolies" (bottom) with those shown in Figure 2 above for July gives a good representation of the dramatic differences in sea surface temperatures and wind patterns that accompany an ENSO warm event. Notice the pattern of warmer-than-normal seawater (up to 2.0 degrees C) in the central and eastern Pacific, and the strong westerly wind anomolies near New Guinea and Micronesia. When such conditions occur together, they can last for months (or even years, as in the most recent case), causing changes in rainfall, storm patterns, and ocean currents throughout the tropical Pacific. This in turn can affect climate in other areas around the globe.

The Pacific ENSO Applications Center has begun to develop similar forms of electronic information about ENSO that can be accessed by computer through the internet. In addition to real-time information such as that shown in these figures, our databases include information about ENSO-related climate variability in specific island areas of the Pacific. At present, these resources can be found by making a connection from a computer with internet access and browsing software (like Netscape, as depicted in these figures) to our network site at "http://naulu.soest.hawaii.edu/" . These resources are under constant development, and our progress in this area will be reported in future issues of Pacific ENSO Update.

Publication of the Pacific ENSO Update is funded in part
by Grant Number NA46GP0410 from the National Oceanic
and Atmospheric Administration (NOAA) Office of Global
Programs. The views expressed herein are those of the
author(s) and do not necessarily reflect the views of NOAA
or any of its sub-agencies.